It would be a big job and there would be a lot of competition for it. Some of the names mentioned over the years have included:
[LIST][*]John Barrowman[*]Vernon Kay[*]Graham Norton[*]Terry Wogan[*]Anton Du Beck[*]Bradley Walsh[*]Paul O'Grady[/LIST]
But it could be a complete surprise. E.g someone like Philip Schofield might be tempted to cross the ocean or, if Dermot O'Leary feels a a bit insecure in Simon Cowell's employment, he might go for it. My guess is that it would be someone younger rather than older. A new kid on the block.
Personally I hope Bruce stays for a while because I think that, once he goes, the show will change in a fundemental way. It may lose it's warmth a bit and become a lot more like those other harsh judging shows out there.